Saturday, March 22, 2014

Sawyer's "remarkably accurate" forecast

Spotted this in the Guardian, not sure what provoked it, but never mind. The claim is that a paper published in 1972 made a remarkably accurate prediction of 0.6C, and this proves we've understood the climate system really well for a long time.

Well.

For starters, it should be pretty obvious that if people published enough random papers, some would (with hindsight) turn out to be close to correct. That in itself would hardly prove prescience, although it might be hard to refute a claim by the lucky one. See also investment analysts who claim to be able to "beat the market"...

But I'm not going to go looking for all the silly forecasts that were wide of the mark, which there surely were - people are still churning them out, remember Lovelock's few remaining breeding pairs of humans, or Bryden's AMOC shutdown, or Keenlyside's cooling? - but merely evaluate the Sawyer paper on its merits. Found courtesy of wmconnolley.org.uk the paper doesn't really seem to have much original research, but repackages other work in what looks more like a commentary. He uses Manabe and Wetherald's climate sensitivity estimate of 2.4C, and a predicted increase in CO2 of 25%, to get a warming of 0.25*2.4 = 0.6C by the end of the (20th) century.

While the final number ended up pretty close, there are a number of assumptions/approximations/errors (take your pick) in that calculation. Firstly, there are other forcings! The IPCC AR5 lists other factors which in total magnitude exceed the CO2 effect, though the positives and negatives broadly cancel. But Sawyer doesn't consider them at all. Secondly, the logarithmic effect of CO2 means that a 25% increase should equate to a 32% of the effect of a doubling, which would work out at 0.8C...not a huge difference in forecast, but a big difference in level of understanding! Lastly (perhaps) there is also the small issue of equilibrium versus transient response - the thermal inertia of the ocean means there's a chunk more warming in the pipeline, probably about a third as much again. All these values have substantial uncertainty even now, of course - and although I'd say the the 2.4C sensitivity value still looks pretty good, others disagree and at best it was a lucky guess to get it right back then.

So all in all, it looks like he made a number of significant errors which end up cancelling out, thus resulting in a forecast that hit the bullseye much more closely than can have been reasonably expected.

13 comments:

A disappointing post. Everybody with some scientific training should immediately recognize that Sawyer's article is indeed a review and that his calculation serves the only purpose to illustrate the ballpark of the greenhouse effect, not to provide accurate numbers. It's also somewhat surprising that the author, despite being a climate scientist, did apparently not know the Sawyer paper. It's a classic.

Well, I wasn't meaning to criticise Sawyer particularly, rather the person who cited the forecast as being remarkably accurate (and therefore evidence of the long-standing knowledge of climate scientists). The fact remains that people since, with (in principle) better knowledge, have give both less accurate forecasts, and also highly uncertain ones. So 0.6C hitting the mark is largely coincidence.

Sorry, but you claimed that Sawyer "made a number of significant errors". This *is* is an unjustified, unfair attack on Sawyer. He just compared warming by CO2 with the natural variation (as far as the science could quantify them at that time). Where should be the error in to doing so? It is just like comparing different forcings!

By the standards of the time, it was quite reasonable, I think. But in terms of our current understanding, there was a lot wrong, and therefore the post-facto accuracy of the forecast was lucky.

Even at the time, the logarithmic effect of CO2 was known (since Arrhenius, if not before). He actually did mention the thermal inertia issue, but didn't seem to recognise how this would (should) affect his prediction. You could I suppose argue that ignoring other forcings was not an error on his part, but rather an error on the Grauniad's part in interpreting his "scenario" as a forecast.

Once more: you (and the bloggers of the Guardian) read way to much into this particular review/comment. Sawyer ignores thermal inertia, because his intention is obviously *not* to make a prediction of the global temperatures in the year 2000, but just to give a ballpark number for the equilibrium temperature increase to be expected upon a ~25% increase in CO2. The final temperature will of course be reached later. This was all well known at that time. The first primitive climate models (which included heat capacities and all that as a matter of course) date back to the early sixties. Syokuro Manabe, the guy who actually worked out the climate sensitivity cited by Sawyer, has a 1969 paper describing a general circulation model with grid cells of ~500 km and 9 atmospheric layers (5 in the oceans), run over 60 years (J. Atmos. Sci. 26: 786). It would be wise to dig a bit deeper into the literature of that time before attempting to judge what they knew and what not.

uups, I'm actually not sure whether you disagree with what I wrote, or not. Do you agree with my point, that the close match of Sawyer's estimate (of 0.6C warming) with the eventual outcome was due to the fortuitous cancellation of several factors which he did not account for? Or do you think it was because he understood the climate system really really well? Or...what?

I entirely agree with you that the 0.6C are more or less a coincidence. My point is just that it was never intended to be a forecast in the sense the Guardian bloggers cite it and in the sense you criticise it. That's the simple reason why Sawyer writes a paragraph on thermal inertia, yet does not use them for his calculation. Sawyer was neither a climatologist nor a pioneer in the field of greenhouse gase, but he was an expert in numerical calculations for weather prediction, and, thus, familiar with maths and physics. He would certainly not have missed something as basic as thermal inertia if this would have been his intention. The researchers of the seventies were not nearly as stupid as you portray them here.

JA: I sort of had you down as a '3cplus or minus a bit' type: when did the shift to 2.4 happen? Was I asleep?BTW - how far away from Richmond are you? It's where I live at the mo...thinking of a pint/cup of tea and cake sometime...

Well, 3C isn't a terrible estimate. However, I do think the most recent and reliable evidence - the combination of limited warming, and more confidence that aerosol forcing is not strongly negative - does point towards a slightly lower value.

We have ventured into Yorkshire, but not as far as Richmond - do you think we'd get out alive? Now the weather's improving, we might start to explore further out of our comfort zone :-)

Well if you look at the paper, the 25% seems to be based on one or other of the two references, but it's probably intended as a rather rough estimate.

As for the 0.6C, this is clearly indicated as relative to pre-industrial, ie the situation prior to the 25% increase, so not specifically 1850 but that's as good a date as any. It's just calculated as a quarter of the equilibrium sensitivity which is obviously an incorrect calculation for all sorts of reasons...